The present invention relates to an assembly using an abrasive strip to machine a cylindrical bearing surface of a workpiece, especially a journal and/or wrist pin of a crankshaft, of the type comprising two opposed jaws which can be clamped against the bearing surface to be machined in order to apply an abrasive strip against the latter while the workpiece is rotated.
Assemblies of this type are known, for example, from Utility Model DE 86 01 817. According to this document, each jaw carries two shoes for applying the abrasive strip, which are mounted elastically on the jaw and each extending over an angle of between 15 and 45.degree. of the circumference of the bearing surface to be machined. This machining assembly has numerous drawbacks among which mention may be made, in particular, of the poor distribution of pressure over the four shoes, the limited angle of contact of the shoes, and therefore of the abrasive strip, with the bearing surface to be machined, and hence the fact that it is impossible to carry out machining which yields not only a good surface finish but also compensates for any defects in the shape of the bearing surface, and the absence of built-in means of checking the diameter of the bearing surface during machining.
Document FR-A-2 702 693 (=U.S. Pat. No. 5,522,762) discloses a machining assembly that involves three shoes for applying the abrasive strip, each extending over an angle of between 60.degree. and 120.degree. and arranged more or less at the three vertices of an equilateral triangle. This machining assembly, despite the advantages it yields over an assembly with four application shoes, is still not satisfactory as regards compensation for defects in the shape of the bearing surface to be machined.
Document FR-A-2 719 516 (=U.S. Pat. No. 5,651,719) relates to a machining assembly which employs the overall structure of the assembly according to the previous document, but is equipped with means of checking the diameter of the bearing surface while it is being machined.
Nevertheless, these known machining assemblies are unable fully to meet all the requirements imposed at the present time as regards in particular the machining of the journals and wrist pins of motor vehicle engine crank shafts.
The present invention is aimed at an assembly that uses an abrasive strip to machine a cylindrical bearing surface of a workpiece, especially a journal and/or a wrist pin of a crankshaft, which assembly, while being of a simple structure, optimally meets the requirements imposed in this field, from the machining precision point of view and that of compensating for defects in shape. The invention is also aimed at an assembly that uses abrasive strip to machine a cylindrical part of a workpiece, incorporating built-in means for checking the diameter of the bearing surface during machining.
The machining assembly in accordance with the invention that uses an abrasive strip to machine a cylindrical bearing surface of a workpiece, especially a journal or wrist pin of a crankshaft, comprises two opposed jaws which can be clamped against the bearing surface to be machined in order to press an abrasive strip against the latter as the workpiece is rotated. A first one of said jaws carries a shoe which has a rigid concave bearing surface in the shape of a sector of a cylinder of a shape that matches that of the bearing surface to be machined, with means for immobilizing an abrasive strip relative to said bearing surface during machining. The second jaw carries two reaction surfaces or pads spaced apart in the circumferential direction of the bearing surface to be machined and directed parallel to the bearing surface to be machined. The layout of the shoe of the first jaw and of the two reaction pads of the second jaw is such that when the jaws are clamped on the bearing surface to be machined, the shoe presses the abrasive strip against the bearing surface to be machined over a circumferential angle which is preferably between 120.degree. and less than 180.degree. and the two reaction pads are pressed directly against the bearing surface to be machined along two generatrices of the latter which are spaced apart by a circumferential angle which is preferably between 60.degree. and 120.degree..
Within the context of the invention, the abrasive strip may be immobilized with respect to the bearing surface of the shoe by adhesive, the strip being stuck to the bearing surface, or preferably by controlled clamping means arranged on either side of the shoe, as close as possible thereto, which allows the abrasive to be renewed simply by moving the strip on between two machining operations.
The two reaction pads may be shoes, but in order to reduce the friction of the reaction pads on the bearing surface to be machined, it is advantageous for use to be made of two rollers mounted on the second jaw so that the axes of the rollers are parallel to the axis of the bearing surface to be machined.
Still with a view to reducing the friction of the reaction pads on the bearing surface to be machined, the reaction pads may have axial mobility with respect to their jaw. Thus, the reaction pads may, by friction, participate in the oscillatory movement in terms of axial translation that the bearing surface undergoes, in a way known per se, in addition to its rotation, while it is being machined by abrasive strip.
The machining assembly according to the invention may further comprise built-in means of checking the diameter of the bearing surface while it is being machined. In this case, in addition to the two reaction pads, the second jaw carries two measuring pads mounted so that they can move on the second jaw in such a way that they can be moved apart and brought closer together so as to be pressed, in the machining position, in diametrically opposed positions, against the bearing surface to be machined. The second jaw further advantageously comprises means for moving the two measuring pads apart when the jaws are not clamped and for bringing them closer together when the jaws are clamped.
Within the context of the invention, said means for moving the two measuring pads apart and bringing them closer together may be actuated directly by the opening and closing movement of the jaws, or alternatively be controlled as a function of this movement for parting them and bringing them closer together.
Two illustrative and nonlimiting embodiments of an assembly in accordance with the invention for machining using an abrasive strip will be described in greater detail below with reference to the appended diagrammatic drawing; in the drawing: